The long reins bring a new contact with the horse. Indeed, when we make long reins we have a contact with the mouth of the horse only. I find that the work with long reins offers a new look, we can see much better the reactions of the horse when we are not on him. I also find that this work creates a different relationship because we have to talk a lot to give indications (walking, trotting, galloping ...) so we see if the horse is attentive or not, if he listens to us. Long reins bring, in my opinion, a confidence without contact other than "hand - mouth", they also bring a complicity when the rider makes requests and the horse answers without hesitation. They are also a good way to learn how the horse's mouth works.
Have a good day,